Optical Design of a Reaction Chamber for Weakly Absorbed Light

This report outlines the possibilities to be found using one or more diffraction-limited high-quality light beams to activate a weakly absorbing gas in a regime where the diffraction spread can be controlled by converging optical devices to within a ratio ofJ2 of the minimum at the beam waist (corresponding lengths between converging elements are within twice the Rayleigh range). Our desigr.s use plane or cylindrical parallel mirrors down which a light beam is repeatedly reflected. In the first design variation,the beam is re-reflected up the parallel mirrors to the entrance aperture where it can be returned repeatedly for a number of multiply reflecting "travels" up and down the parallel mirror reaction chamber. In the second variation,the return of the beam after each multiply reflecting "travel" down the chamber is external to the chamber and is achieved by two mirror reflections. For diffraction control the return mirrors can be made converging. For multiple laser excitation, any of the external return mirrors can be replaced by a laser. The advantages of these designs are a high degree of uniformity of chamber illumination with a reasonably high number of passes. Drawbacks of the designs are the large space needed for beam return (many ten* of meters for some parameters) csnd (common to all high optical quality chambers) the figuring and reflectivity demands on the mirrors.